Electroplating is one of several critical back-end IC packaging processes, in which a direct current is applied between the edge of a wafer, which serves as an anode, and a plating window at a center of the wafer, which acts as a cathode, and heights of resulting metal bumps are controlled by adjusting the magnitude of the current and the concentration of a plating solution in a plating tank.
Due to the non-conductive nature of photoresist, it is necessary to remove photoresist near the wafer edge before the electroplating process. The width of photoresist to be removed depends on a wafer edge exclusion (WEE) width of a previous exposure process. There are a large number of traditional WEE methods which can be generally divided into two groups: chemical WEE and edge exposure. Chemical WEE is accomplished by, during photoresist coating of a wafer, spraying a solvent on its edge so as to remove photoresist coated thereon. Such an approach suffers from several drawbacks such as a long process cycle, high cost due to the expensive solvent, and proneness of the solvent to be sprayed on the central pattern area of the wafer and thus significantly degrade pattern quality. In an edge exposure method, a wafer is vacuum-absorbed onto a rotary table, with a set of UV exposure lenses being arranged above the edge of the wafer and each configured to produce a uniform light spot of a certain size. Rotation of the rotary table allows the edge of the wafer to be exposed. Compared to chemical WEE, edge exposure offers a variety of advantages such as higher productivity, lower equipment cost and easier process control and etc.
During edge exposure, the wafer is pre-aligned after it is transferred onto the rotary table in order to eliminate possible errors occurring during the transfer that is conducted in a random manner, so as to achieve centering and notch orientation of the wafer. The centering is a process to move the wafer so that its centroid coincides with a centroid of the rotary table. The orientation is a process to rotate the wafer to align its notch with a predetermined position. These processes ensure the wafer to be exposed on the exposure table at a fixed position. The pre-alignment is a precise positioning of the wafer prior to the edge exposure, and its precision has a direct impact on the efficiency of the whole wafer processing apparatus.
Recently, the requirements from the market on pre-alignment and edge exposure, as well as on their degrees of automation, are getting more and more critical. Pre-alignment capabilities are required to handle not only a variety types of wafers such as through-hole wafers, warped wafers and ultra-thin wafers but also 6/8/12-inch wafers. Edge exposure capabilities are required to enable not only edge exposure, annular exposure, segment exposure and etc. but also exposure field of view (FoV) adjustment and exposure energy monitoring. At the same time, lowering the cost of wafer processing apparatuses is also in demand.
In the state of art, wafer pre-alignment and edge exposure are usually accomplished using two separate devices equipped with respective independent control systems, which take up large space. In addition, as there are many objects that need to be controlled and simultaneous control of motion axes including switching, rotation, lifting and centering axes is needed, the associated pre-alignment methods are tedious and the systems are complex, high-energy consuming and costly.